Hello and welcome to Just answer.com, Super mechanic here. About your vehicle.
There has been a service bulletin for that type of concern.
You will want to have the transmission checked by a trans shop or dealership to have an estimate of the repairs.
Here is what it states.
Bulletin No.: 03-07-30-007E
Date: May 05, 2008
with 4L60-E (RPO M30) or 4L65-E (RPO M32) Automatic Transmission
Park/Neutral to Reverse or Drive (commonly called garage shift) engagement times may be different from what many customers might be accustomed to especially if they have recently traded for a General Motors vehicle.
Normal Park/Neutral to Reverse or Drive engagement time will be in the vicinity of one (1) second if the automatic transmission fluid (ATF) is above 0°C (32°F). When the ATF is colder than 0°C (32°F), the engagement time will become longer the colder the transmission fluid. A time limit factor cannot be placed on proper engagement times because the factors that affect transmission operation are many.
At the moment of engine start, for economy and emission requirements, the transmission line pressure is maintained at low levels. This allows a lower cold idle engine RPM (the colder the oil is, the higher the energy level (engine torque output) required to pressurize the oil).
When the driver moves the shift lever from "Park/Neutral" to "Reverse" or any "Drive" range, the following actions must occur for the powertrain control module (PCM) to determine at what pressure to control the transmission line pressure.
The module will look at the following items:
When the module determines the status of the above items, the module will apply the appropriate signal to the PWM pressure control solenoid, thereby controlling the application of the required clutch(es) depending on the driver's selection.
Controlling transmission apply pressures allows the transmission to engage in a smooth manner, thus reducing the potential of abrupt/harsh engagement of the transmission.
All of the above actions happen very quickly; however, the resultant time interval may be approximately one second.
When determining if shift times are excessive, the exact conditions that the vehicle was in at the time the extended garage shift occurred must be duplicated. The following are several possible conditions.
No attempts to repair should occur unless the condition can be duplicated under the conditions the customer experienced.
Road test the vehicle following the steps below until the vehicle engine and transmission temperatures are at normal operating temperatures. This is approximately 16 km (10 mi) of driving.
If an out of line condition (excessively long time, over two seconds, to engage a gear) is observed, the following components within the transmission should be inspected for the cause of the concern.